1. Field of the Invention
The present invention is directed to a vertical magneto-optical head for reading data from a magnetic recording medium, and in particular to a shielded vertical magneto-optical read head.
2. Description of the Prior Art
The basic principles of operation of a magneto-optical head for reading data stored on a magnetic recording medium are described in xe2x80x9cmagneto-optical thin film reading head,xe2x80x9d Hatakeyama et al., Applied Optics, Vol. 25, No. 1 (Jan. 1, 1986), pp 146-150. Such a magneto-optical read head is used for detecting magnetic transitions, arising from magnetic fields respectively associated with zeroes and ones in a magnetic storage medium. The magneto-optical read head has a region thereof which is sensitive to such magnetic transitions, such as by changing its polarization or other optical properties dependent on the magnetic field associated with the magnetic transition in the magnetic recording medium.
The magnetic transitions occur in succession along the length of the magnetic recording medium and a problem associated with the magneto-optical recording head disclosed in the Hatakeyama et al. reference is that when responding to the magnetic field associated with the magnetic transition closest to the sensitive region of the head, magnetic fields associated with adjacent transitions in the recording medium can be strong enough to influence, and thereby falsify, accurate detection of the closest magnetic transition. This requires the magnetic transitions on the recording medium to be separated by a certain distance from each other so that when the magneto-optical recording head reads or detects the closest magnetic transition on the recording medium, it is not unduly influenced by the magnetic fields associated with adjacent transitions. This minimum spacing between magnetic transitions means that the linear resolution associated with a magneto-optical head of the type described in Hatakeyama et al. is low. This can be seen from FIG. 8 therein, which shows a practical limit of the linear density as being below 500 bits/mm (i.e., a bit size of more than two xcexcm). This is more than one order of magnitude too low for competing with currently available commercial products.
To improve the linear density, it is known to introduce a single shield into the structure of a magneto-optical recording head, as described in U.S. Pat. No. 5,568,336 and French Patent 2 797 708. The magneto-optical heads described in those references are formed by a number of successive layers on a substrate, and one of these layers is composed of shield material. The shield material is opaque, and is disposed only on one side of the head, because the other side must be light-transmitting, to allow light to enter into and exit from the gap material and the sensing layer, which is the layer that changes it""s optical properties dependent on the presence of a magnetic field. Although this one-shielded design has an improved linear density compared to the unshielded design described in Hatakeyama et al., the linear density associated therewith is still not sufficient for current systems. Another problem associated with this design is that it produces an asymmetric pulse response due to the magnetic structure thereof being asymmetric by design. This makes this single shielded head less desirable in modern read detection channels, such as Partial Response Maximum Likelihood (PRML) channels.
It is also known from the aforementioned French Patent 2 797 708 to provide additional elements (i.e., additional material) at the magneto-optical head in order to support the recording medium and thereby reducing spacing loss during the read-out process.
Another type of known read head is a magneto-resistive head. It is known to construct magneto-resistive heads with a single shielded design as well as with a double shielded design, as described in xe2x80x9cmagneto-resistive heads: fundamentals and applications,xe2x80x9d Mallinson (1996) pages 66-69. Double shields, i.e., a magnetic shield on each side of the magneto-resistive layer, can be used in a magneto-resistive head, because the problem of blocking light transmission is not present. Such double-shielded structures were introduced to increase the capability to read data stored with increased linear density, while also maintaining a symmetrical response. Double shielded magneto-resistive heads are currently the most widely used configuration for tape heads in the most recently available commercial products.
It is an object of the present invention to provide a magneto-optical read head with the capability of reading data having an increased linear density in comparison to currently available magneto-optical read heads. It is a further object of the present invention to provide a magneto-optical read head which is capable of reading data with a linear density comparable to that readable by a double shielded magneto-resistive head.
The above object is achieved in accordance with the principles of the present invention in a dual shield magneto-optical read head having a layer structure, wherein a magnetic shield layer is provided at a side of the sense layer through which light enters and exits, with this shield layer being composed of material which is transparent to light at the employed wavelength.
By employing a magnetic shield layer which is transparent to light at the employed wavelength, magnetic shielding can be provided on both sides of the sense layer, thereby shielding the sense layer from stray magnetic fields on both sides, so as to improve the linear density reading capability. Moreover, since shielding layers are present at both sides of the sense layer, the pulse response asymmetry is reduced, thereby making the magneto-optical read head suitable for use in systems employing detection channels of the type that require a symmetric pulse response. The material forming the transparent shield layer is soft magnetic (high permeability) material which prevents magnetic flux, such as arising from a magnetic transition in the recording medium, from entering into the sense layer, except when the magnetic transition is located between the transparent shield layer and the other shield layer at the opposite side of the sense layer. As noted above, the material is transparent to light at the given wavelength which is employed for read-out.
The shield material in the transparent shield layer is preferably composed of garnet having a composition A3X5O12, wherein A is one or more of Y, Bi, or any rare earth element, X is one or more of Fe, Ga, Al, In, Sc, Sm or Nd. Examples of suitable compositions are (Lu2.5Bi0.5)(Fe5)O12 and (Tm2.5Bi0.5)(Fe4Ga1)O12.
The material forming the transparent shield layer should have approximately the same index of refraction as the substrate material of which the head is formed. The transparent shield layer also preferably has a low Kerr/Faraday rotation. The thickness of the transparent shield layer will depend on the permeability of the material forming the layer, but will typically be in a range between 1 and 2 xcexcm.